Studies from the Laboratory of Biochemistry (NHLBI) have shown that there is an accumulation of oxidized proteins with age and in some disease states. These damaged proteins are degraded by the multicatalytic protease (MCP), a multicatalytic proteolytic complex found in eukaryotic cell. In order to gain insight into the mechanism of damaged protein degradation, we have started to examine this process with two enzymes, E. coli Glutamine Synthetase (GS) and L. mesenteroides Glucose-6- Phosphate Dehydrogenase (Glu-6-PDH), that have been modified by different ways: metal catalyzed oxidations, treatment by ozone and reaction with 4-hydroxy-2-nonenal (HNE), a major lipid peroxidation product. These damages which inactivate the enzymes lead in most cases to an enhanced susceptibility of the modified protein for proteolysis by the MCP. However, inactivation with HNE results only in a small increase of proteolytic susceptibility with GS and no increase with Glu-6-PDH. In the case of the latter enzyme, we have shown that prolonged incubation with HNE leads to the appearance of cross-linked protein that is resistant to proteolysis and acts as a potent inhibitor of the MCP. Oxidized Glu-6-PDH is readily degraded by the MCP. However, treatment of oxidized Glu-6-PDH with HNE also leads to the formation of cross-linked protein that is resistant to proteolysis and inhibits the MCP. In an effort to characterize the mechanism by which the MCP is inhibited by HNE modified protein, the structural patterns which confer the inhibitory function and the kinetics of inhibition have been investigated. Inhibition of MCP by HNE cross-linked protein is noncompetitive with respect to oxidized glutamine synthetase substrate. As a noncompetitive inhibitor, HNE cross- linked protein would therefore be an even better candidate for promoting the accumulation of oxidized protein since noncompetitive inhibitors are not sensitive to substrate concentration.